The present invention relates to heat pipes, and more particularly to an improved heat pipe adapted for use in converting solar energy into thermal energy or heat.
Heat pipes adapted to transfer heat from one area of heat input to another area of heat dissipation have long been known in the art. Because of their high efficiency in transferring relatively large quantities of heat with small temperature gradients, heat pipes have been used to absorb solar energy, convert the solar energy into thermal energy or heat, and transfer the heat to a heat transfer or storage fluid. An example of a prior art solar heat pipe is shown and described in an article entitled, "Research Applied to Solar Systems", pages 1-18 of "Proceedings of the Solar Thermal Conversion Workshop", Arlington, Va., Jan. 11-12, 1973, sponsored by the National Science Foundation Grant GI-32488.
The known type of heat pipe adapted for use in converting solar energy into thermal energy is shown schematically in FIG. 1. A hermetically sealed chamber or envelope ENV of tubular form has evacuated therefrom all noncondensable gases. The envelope is made from a heat-conductive material, more specifically from metal, and has an outer surface portion provided with a coating for absorbing solar radiation. The portion of the envelope adapted to absorb solar energy comprises an evaporator region E. The envelope has another portion in contact with a heat transfer fluid or medium adapted to remove the heat generated in and delivered from the evaporator region, such portion being known as a condensor region C.
The interior of the envelope ENV normally contains a wick W along its entire length which comprises a cylindrical cluster of wires, glass fibers, or cloth, or a hollow porous powdered or ceramic body, or a radially extending, equally spaced plurality of fins or threads spirally extruded or formed on the interior wall of the envelope. In the evaporator region of the envelope, the wick W is saturated with a working fluid. The wick W surrounds or is provided with a central vapor duct or free space V, which is shown to be centrally located within the wick; and, when solar radiation is not being applied to the evaporator region, the vapor duct V contains only the vapor of the working fluid at a pressure which corresponds to the saturation pressure of the fluid at an isothermal temperature.
When solar energy, in the form of rays R, is absorbed by the coated outer surface of the evaporator region E, the outer extent of the envelope is heated by the transformation of radiation in at least a portion of the visible range of the solar spectrum to lattice vibrations in the solid outer surface i.e. heat. The heat is thereafter thermally conducted through the wall of the envelope and into the wick.
The conducted heat is absorbed by the working fluid and this causes a portion of the working fluid to evaporate. The resulting vapor is at a substantially higher pressure and migrates through the vapor duct V toward the cooler condensor region C, at the other end of the envelope ENV. A heat transfer or storage fluid is pumped or directed to flow over the outer surface of the condensor region C of the envelope. The vapor condenses on the inner wall of the envelope at the condensor region; and as the vapor condenses, it gives up heat in the condensor region, which was acquired earlier during vaporization in the evaporator region. The heat output is represented by arrows Q.
The wick W is provided with capillary-sized vessels for pumping or conveying the condensed working fluid from the condensor region C to the evaporator region E. Although it may be possible to convey the condensate solely by capillary action within the wick, heat pipes used for solar energy conversion are of such length that it has been found preferable to incline the evaporator region E downwardly relative to the condensor region C to gravitationally augment the flow of the condensate. With such inclination, a substantial portion of the condensate flows through a lower portion of the vapor duct and is distributed transversely by capillary action within the evaporator region E into those portions of the wick from which the working fluid has evaporated.
The phase transformation of working fluid from its liquid state to its vapor state and back again and the capillary transport of the working fluid will continue as long as the evaporator region E receives a substantial amount of solar radiation.
As previously stated, the known type of heat pipe used to absorb and convert solar energy has used an envelope made from metal and has relied upon the absorption of solar energy in the outer surface of the evaporator region and the conduction of heat from the outer surface inwardly through the envelope wall. A shortcoming of such heat pipes is that some of the heat generated in the outer surface will be lost by virtue of the thermal resistance of the wall of the envelope and of the interface between the wall and the wick.
To reduce the temperature drop across the wall, it is possible to reduce the wall thickness of the metal envelope. However, the pressure of the vapor developed within the envelope and temperatures generated within the envelope wall provide substantial limitations on the minimal wall thickness. That is, the wall must be thick enough to withstand creep or deformation at high temperatures and internal pressures. It is possible to select working fluids which develop lower vapor pressures, but the field of choice of fluids is limited by their chemical stability and chemical compatibility with the wick and wall materials, phase transition temperatures, and also by their heat of vaporization.
It is also well known that the envelope may be made from materials which have high thermal conductivity. However, many envelope materials having high thermal conductivity are difficult to form or fabricate at long lengths suitable for solar absorption and conversion. For example, it is relatively difficult to form durable, leak-proof seals between tubular sections of such materials and also between tubular sections and end caps made from such materials. Moreover, the choice of envelope materials is restricted by the requirement that the envelope, the wick, and the working fluid be compatible at high operating temperatures. For example, the materials must be chosen to avoid the possibility of galvanic corrosion and the formation of non-condensable gases.
An object of the present invention is to provide an improved heat pipe applicable for use in converting solar energy into heat energy which eliminates the shortcomings or problems associated with conventional heat pipes.
It is yet another object of this invention to provide an improved heat pipe device which is capable of being mass-produced from low cost materials and yet which has a durability at least equivalent to conventional solar heat pipe devices. It is a further object of the present invention to provide a heat pipe device which is subject to minimal convective, conductive, and emissive heat losses.